PERMANENT PRESENCE OF GRAZING-RESISTANT BACTERIA IN A HYPERTROPHIC LAKE

The size structure of planktonic bacteria from a hypertrophic lake was investigated at 5- to 15-day intervals by means of a semiautomatic im age analysis system during 1 year. Characteristic of this bacterial as semblage was the permanent presence of large filamentous bacteria and small cocci with cell sizes of < 0.01 mu m(3). These filamentous bacte ria, sometimes longer than 200 mu m and with cell volumes of up to 276 mu m(3), are larger than nanoflagellates (< 20 mu m) and, even, metaz oans living in the lake. Although they account for only 4 to 16% of ba cterial abundance, their contribution to total bacterial biovolume was between 45 and 86%. An analysis of the food web structure indicates t hat this particular bacterial size structure may be the consequence of a strong bacterivory pressure by nanoflagellates and the absence of o ther larger bacterivores. The persistence of bacterial forms resistant to grazing has important consequences for the carbon flow within the microbial food web.